var stream = require("stream");
var Readable = stream.Readable;

//Readable可以进行实例化
var rs = new Readable();
var c = 'a'.charCodeAt(0);//UNICODE码
//_read
rs._read = () => {
    if(c < 'z'.charCodeAt(0)){
        rs.push(String.fromCharCode(c++));
    }
    else{
        rs.push(null);
    }
}
//将可读流输出一个可写流
rs.pipe(process.stdout);